Dependabot: add a 30-day cooldown between go dependency updates (#8312)

Documentation for the "cooldown" config parameter is here:
https://docs.github.com/en/code-security/dependabot/working-with-dependabot/dependabot-options-reference#cooldown-

Author: aarongable

.github/dependabot.yml

@@ -14,6 +14,8 @@ updates:
     schedule:
       interval: "weekly"
       day: "wednesday"
+    cooldown:
+      default-days: 30
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule: